1. Field of the Invention
The present invention relates to the stabilization of the optical output characteristics of a semiconductor laser and, in particular, to an apparatus for controlling a laser drive signal such that the optical output level of the laser remains substantially constant irrespective of temperature changes.
2. Discussion of Background
Recently, semiconductor laser diodes have been proposed as an electro-optical converter for use in optical communication systems to convert digital or analog electrical signals into optical signals, which are then transferred to data receiving units associated therewith through a signal transmission cable such as a known optical fiber. A problem in such application is that the optical output characteristics of a semiconductor laser change with temperature. The output characteristics which can vary are the threshold level of laser oscillation and the differential quantum efficiency.
To compensate for temperature changes in the laser output, it is required that the drive pulse signal should be effectively controlled with respect to both the biassing level of a bias current and the amplitude of drive pulses applied to the laser. In the conventional systems, however, the above technical requirements cannot be fully satisfied sufficiently, in particular, in a highspeed optical data communication. This results in that temperature changes in the optical output level of the laser cannot be compensated effectively when it continues to emit optical data signals at high speed, for example, at a data transmission rate of more than several hundred megahertz (MHz).
In Japanese patent application No. 55-83280, there is disclosed a laser output stabilizing apparatus including a device for detecting the peak level of a monitoring signal produced by a photodiode to represent the actual laser output, and a device for detecting an average value of the monitoring signal. The biasing level of a bias current for the semiconductor laser may be adjusted on the basis of the average value detected, while the amplitude of the drive pulse voltage of the bias signal is modulated in response to the detected peak value of the laser output. In such arrangement, however, it is very difficult to perform the temperature compensation if the data transmission rate is increased to reach the above-identified high level. This is because, under such condition, the peak detector cannot detect the peak level of a high-frequency laser output signal with sufficient detection accuracy.